The MRI apparatus is an apparatus which measures an NMR signal generated by an object, especially, nuclear spins which form human tissue, and images the shapes or functions of the head, abdomen, limbs, and the like in a two-dimensional manner or in a three-dimensional manner. In the imaging, different phase encoding is given to NMR signals by the gradient magnetic field and frequency encoding is also given to the NMR signals, and the NMR signals are measured as time-series data. The measured NMR signals are reconstructed as an image by two-dimensional or three-dimensional Fourier transform.
The MRI apparatus needs to emit a high frequency magnetic field pulse (RF pulse) to the human body in order to generate an NMR signal. Heat is generated when the human body absorbs the RF pulse. The amount of absorption of RF pulses per unit time and unit mass is called a SAR (Specific Absorption Rate). According to IEC 60601-2-33, 2nd edition, SARs to be controlled are whole body SAR, partial body SAR, head SAR, and local SAR, and these are defined by (Expression 1), (Expression 2), (Expression 3), and (Expression 4), respectively.
                    [                  Expression          ⁢                                          ⁢          1                ]                                                                      Whole          ⁢                                          ⁢          body          ⁢                                          ⁢                      SAR            ⁡                          [                              W                ⁢                                  /                                ⁢                kg                            ]                                      =                              E            ⁡                          [              W              ]                                            M            ⁡                          [              kg              ]                                                          (                  Expression          ⁢                                          ⁢          1                )                                [                  Expression          ⁢                                          ⁢          2                ]                                                                      Partial          ⁢                                          ⁢          body          ⁢                                          ⁢                      SAR            ⁡                          [                              W                ⁢                                  /                                ⁢                kg                            ]                                      =                              E            ⁡                          [              W              ]                                                          m              p                        ⁡                          [              kg              ]                                                          (                  Expression          ⁢                                          ⁢          2                )                                [                  Expression          ⁢                                          ⁢          3                ]                                                                      Head          ⁢                                          ⁢                      SAR            ⁡                          [                              W                ⁢                                  /                                ⁢                kg                            ]                                      =                                            E              h                        ⁡                          [              W              ]                                                          m              h                        ⁡                          [              kg              ]                                                          (                  Expression          ⁢                                          ⁢          3                )            Local SAR [W/kg]=energy absorbed into arbitrary 10 g per unit time  [Expression4]
Here, E indicates the amount of absorption of RF pulses per unit time, M indicates the mass of an object, mp indicates the object mass of a portion to which an RF pulse is emitted, Eh indicates electric power of RF pulses absorbed into the head, and mh indicates the mass of the head. For the four types of SAR described above, upper limits are shown in NPL 1, and it is necessary to comply with them.
In the MRI apparatus, it is also necessary to increase the frequency of an RF pulse applied according to an increase in the static magnetic field strength. Electric power of absorbed RF pulses is proportional to the square of the frequency of the RF pulse. Accordingly, particularly when developing a high-magnetic-field MRI apparatus, it is very important to estimate the SAR accurately.
Regarding the estimation of the SAR, a method of calculating the amount of absorbed RF pulses is disclosed in NPL 1. In NPL 2, the amount of absorbed RF pulses is calculated by solving the Maxwell equation approximately. PTL 1 discloses a method of calculating the whole body SAR, the partial body SAR, and the head SAR accurately by changing the model of the object according to the object data, an imaging part, or an imaging parameter.